Combustion catalyst for solid ammonium nitrate propellant grain



United States Patent 3,498,856 COMBUSTION CATALYST FOR SOLID AMMONIUMNITRATE PROPELLANT GRAIN Samuel W. Harris, Chicago, 11]., assignor toStandard Oil Company, Chicago, 111., a corporation of Indiana 7 NoDrawing. Filed July 24, 1967, Ser. No; 655,328

Int. Cl. C06d 1/00 US. Cl. 149-19 Claims ABSTRACT OF THE DISCLOSUREPropellant grain containing a major amount of am monium nitrate as theprimary oxidizer, polymeric olefin as the primary binder and an'effective amount of combustion catalyst comprising a hydrocarbon havinga boiling point of between 150 C. and 500 C. and which is capable offorming a peroxide at a temperature in the range of the combustion ofthe propellant.

BACKGROUND OF THE INVENTION This invention relates to a new class ofpropellant compositions more particularly to ammonium nitratecompositions having a polymeric olefin binder and a novel combustioncatalyst.

Ammonium nitrate is' classified as a high explosive even though it isextremely insensitiveand cannot be readily ignited by the localapplication of heat or by a blasting cap. Even when ignited, ammoniumnitrate alone does not conistently sustain the propagation ofcombustion. In order to utilize the excess oxygen available from thedecomposition" of ammonium" nitrate, 'oxidizable' materials" are mixedwith the ammonium nitrate. Generally, these mixtures of ammoniumnitrateand oxidizable material are more sensitive thanthe ammonium nitratealone, but the mixtures are also insensitive and slow burning. Theutilization of certain binder materials, which provide excellentstability and good ballistic properties, generally create even moreinsensitive compositions due to thestability. Such a class of bindersystems, as for example,

polymericolefins, e.g. polyethylene,-and polyethylene de-- rivatives,are highly'desirable, but incertain. applications, it is necessary toincorporate into the composition a combustion .catalyst'which offerscontrol of the burn rate.

One requirement forasolid propellant suitablefor military use is that itbe ballistically stable after long periods of storage at temperatures ashigh as 160 F. and as low as -'65 F. Anotherrequirement is thatthe'burning of the composition following storage at either of theseextremes, be 'unireiar'ana not erran aiiiarnany"near the burningcharacteristics of such composition which has not been subjected tothese conditions. The proper choice of a combustion catalyst, therefore,requires a consideration of the ability of the catalyst to .aid in comrbustion only 'when called upon to do so. For this reason, it isnecessary for the catalyst to be stable at all temperatures common tothese storage conditions, but readily active at the initiation ofcombustion upon ignition.

SUMMARY OF THE INVENTION tion and in the claims is intended to meaneither ordinary commercial grade ammonium nitrates such asconventionally grained ammonium nitrate containing a small amount ofimpurities which may be coated with a small amount of moisture-resistingmaterial such as petrolatum or paratfm, or to mean military gradeammonium nitrate or mixture of minor amounts (usually less than 10%) ofother organic or inorganic nitrates such as guanidine nitrate or sodiumnitrate or potassium nitrate with the ammonium nitrate in a mixture offinely ground and unground or coarsely ground ammonium nitrate ispreferred. The major proportion of the ammonium nitrate should be finelyground, but it is well known in the propellent art that particle sizeand size distribution within the composition are related to theballistic properties. The amount of grinding then, will depend upon thedesired properties of the propellant.

The polyolefin binder material may be any polymeric olefin known to theart as a binder material for use in ammonium nitrate based solidpropellants, for example, polyethylene; Surlyn A type polymers (which isthe subject of copending application, S.N. 655,300 filed July 24, 1967);etc. The Surlyn A compositions may be obtained commercially from E. I.du Pont de Nemours and Company.

The combustion catalyst is a hydrocarbon-containing compound having atleast one active site capable of forming a peroxide radical attemperatures in the range of the combustion of the composition intowhich the catalyst is incorporated. Substantially all active sites arereacted at said temperature. However, the catalyst should be stable,i.e. exhibit substantially no peroxide formation, at temperatures towhich the grain is subjected during storage. It is preferred that theactive site be capable of oxidation to form the peroxide radical. Thecatalyst, therefore, should undergo substantially complete oxidation toform peroxide at a temperature in the range of approximately 125 C. to250 C., and substantially no oxidation below about 125 C. The catalystmust have a boiling point in the range of 150 C. to 500 C., in order toprovide the desired volatility. It is preferred that the catalystcontain .also contain atoms other than carbon and hydrogen, for

tertiary hydrogen, since these compounds are known to readily formperoxides. Exemplary of the compounds suitable for use in this inventionare partially hydrogenated naphthalenes, e.g. tetrahydronaphthalene;paritially hydrogenated anthracenes, e.g. octahydroanthracene;

cumene; substituted cumenes, e.g. phenylcumene; polybutene containingtertiary hydrogen and having a molecular weight above 300, preferably inthe range of 300 to 2,000; etc. The compounds suitable in this inventionmay example, nitrogen and nitrogen-containing groups.

It is also desired that the composition contain a carbon component. Thefinely divided carbon may be added in amounts up to 10% by weight of thecomposition, preferably in amounts of 1 to 5% by weightforthe purpose ofimproving the burning rate of the composition. Highly adsorptiveactivated carbons such as Norite and Nuchar well known inthe art asactivated carbon of vegetable origin, make up one class of efiectiveburning rate components. A second general class of carbon useful forincreasing the burning rate of compositions are the carbon blacks,roughly classified as the channel blacks and contain adsorbed hydrogenand oxygen. Bead type carbon blacks, such as Micronex Beads and StatexBeads, v

are also suitable. A third type of carbon which may be used is finelyground petroleum coke, particularly petroleum coke obtained as residuein the pipe-stilling of midcontinent heavy residuums.

The oxidizer and binder should be present in the composition inessentially stoichiometric amounts, in order to best utilize the excessoxygen obtained from the oxidizer, and an effective amount of catalystmust be present. A major amount of oxidizer should be present. Suitablepropellent compositions comprise:

Percent by weight (a) Binder 5 to 40 (b) Catalyst 0.5 to 5 (0) Carbon 0to (d) Remainder of composition essentially ammonium nitrate.

Preferred compositions are as follow:

Percent by weight (a) Binder to (b) Catalyst 1.5 to 3.5 (0) Carbon 1 to5 (d) Remainder of composition essentially ammonium nitrate.

Other additives which may be added are present in an amount sufficientto provide effective improvement of the properties for which they areadded.

In preparing the compositions of this invention, any procedure known tothe art for the preparation of ammonium nitrate grains containing athermoplastic binder may be used.

With respect to the properties of a solid propellant, the velocity atwhich a solid propellant is consumed during operation is called theburning rate. It is measured in a direction normal to the propellentsurface and is usually expressed in inches per second. The burning ratemay be expressed by the following relation, in which the influence ofall performance parameters is small compared to the chamber pressure andthe initial grain temperature.

The burning rate or velocity of propellent consumption r is usuallygiven in inches per second; the chamber pressure p in pounds per squareinch; a and n" are constants. The constant a varies with the initialpropellent temperature, and thus the burning rate is a function of thetemperature of the grain prior to combustion. The lower the value of n,the less is the detonating character of the decomposition of a gasproducing composition and the more even and smooth is the burning rateof the propellent grain. Thus, a sustained thrust rather than adetonation is obtained by smooth burning of the grain.

The temperature sensitivity for different solid propellants is usuallyexpressed as the percentage change of thrust per unit of temperaturechange. Temperature changes effect the equilibrium pressure and theburning rate. The definitions of the temperature coefficients are givenby Sutton, Rocket Propulsion Elements (2nd ed. 1958).

Here 'n' is the temperature sensitivity coefficient of equilibriumpressure at a particular value of K (K is the ratio of the burningsurface to the throat area), expressed in percent pressure change perdegree temperature change. Mathematically it is defined as the partialderivative of the natural logarithm of the equilibrium chamber pressurep with respect to temperature T. The other temperature sensitivitycoeflicient a refers to the change in burning rate r of a solidpropellant with respect to temperature T at a particular value ofchamber pressure p It is also known as the burning rate temperaturecoefficient, while 'lr is known as the temperature sensitivity ofpressure.

For most propellent applications, as low a temperature coefficient aspossible is desirable and even required for engineering designconsideration. Lower pressure levels over a given temperature levelallows a sizable weight savings for most missile applications.

DESCRIPTION OF PREFERRED EMBODIMENTS Weight Example Formulation PercentAmmonium nitrate Carbon black III Carbon black Surly nit--. Cumene TheSurlyn A of the above formulations have a softening point of about C.,and contain sodium as the primary metallic ion. All of the aboveformulations were molded into grains and cut into strands for testing ina Crawford bomb. The results of the tests are shown in the table below.

TABLE Ballistic Properties Percent, F.

Product of Example n ap 1rx r1000 (hr/sec.)

What is claimed is:

1. A propellent composition comprising:

(a) a major amount of ammonium nitrate;

(b) a binder selected from the class consisting of polyethylene, and apolymeric material having a softening point in the range of from 80 C.to C., said polymeric material comprising a crossed-linked thermoplasticresin contaniing:

(i) a major amount of ethylene units; and

(ii) interchain forces developed between carboxyl groups pendant from analpha olefin chain and metallic ions selected from the group consistingof sodium, potassium, calcium, magnesium, and Zinc; and

(c) from 0.5 to 5.0 weight percent of a combustion catalyst selectedfrom the class consisting of partially hydrogenated naphthalenes,partially hydrogenated anthracenes, cumene, substituted cumenes,polybutenes containing tertiary hydrogen and having a molecular weightgreater than 300.

5 6 2. The composition of claim 1 wherein said binder is ReferencesCited present in the range of 5 to 40 weight percent; said catalyst ispresent in the range of 0.5 to 5.0 weight percent; 7 UNITED STATESPATENTS carbon is present in the range of 0 to 10 weight percent;$147,161 9/ 1964 et a1 14919 and the remainder of said composition isessentially amr 3,305,523 2/1967 Burnslde 14919 X 0 3,338,764 8/1967Evans 14919 monium nitrate.

3. The com sition of claim 1 wherein said catal st is tetrahydmapggmeney BENJAMIN R. PADGETT, Primary Examiner 4. The composition of claim 1wherein said catalyst is octahydroanthracene. 10 149 20 5. Thecomposition of claim 1 wherein said catalyst is phenylcumene.

